This invention relates to wrapping objects with thermoplastic film, such as polyvinyl chloride wrapping film and, more particularly, to apparatus for making a tear-resistant hole in such film.
It is known in the art that items to be packaged are frequently surrounded and sealed in thin plastic film. The sealed item is subsequently placed in a heating device in order to shrink the film. Ideally, the film will closely conform to the shape of the item being packaged. In order for this to occur, air trapped within the sealed film enclosure must be given a way to escape. This is usually done by putting a small hole in the film prior to the sealing operation.
It has been known in the art to employ a mechanical punch for making a small hole in the film prior to the sealing operation. However, some films are prone to tearing at the hole due to the high tensile forces on the film during the shrinking operation. It is preferred to melt a hole in the film with a device which has been heated to a temperature sufficiently high to melt the film. This results in a hole having a peripheral edge which resolidifies into a beaded configuration which is not conducive to tearing. The problem, however, is that the temperature required to melt thermoplastic film is at a level on the order of 250.degree. F. which may cause harm to an operator.
The D. K. Christian U.S. Pat. No. 3,525,279 discloses an electrically heated punch for providing a tear-resistant deflation hole in thermoplastic film so that air trapped by the film during wrapping may escape so as to thereby permit the film to shrink snugly around the article to be packaged. Christian's electrically heated punch is constantly maintained at a temperature above the melting point of the thermoplastic film and employs a film support plate acting as a heat sink and/or a fan for blowing air in an attempt to remove heat from the process. Still further, Christian provides no shielding for protecting an operator from being harmed by the electrically heated punch.